The present invention relates to the field of polishing, and especially to chemical mechanical polishing. More specifically the present invention is directed to improvements in retention of the polishing surface and retention of polishing fluids during polishing.
Polishing a workpiece to produce a mirror-like, defect-free surface has applications in many fields of endeavor. Such polishing processes have become extremely important and widespread, for example, in the fabrication of semiconductor devices. The critical step of polishing a semiconductive wafer or substrate is required at a number of different stages along the varied processes employed to fabricate semiconductor devices.
Chemical mechanical polishing is rapidly becoming a technique of choice for polishing substrates, and particularly for use in the manufacture semiconductor devices on a wafer or substrate. The devices are connected by a process generally referred to as metalization, in which connecting lines of metal, often aluminum or copper, are applied by vacuum deposition or other suitable processes.
The performance level of semiconductor devices employing a conventional single metal layer connecting the devices is fast becoming unsuitable. Modem, high performance devices utilize multilevel metal interconnections. Multilevel connections may be constructed by depositing a dielectric or insulating layer over a first metal layer, etching via holes throughout the dielectric layer, and then depositing a second metal layer which fill the via holes to connect with the first metal layer. These devices offer higher device density and shortened interconnection lengths between the devices.
Since each of these metal and dielectric layers have an appreciable thickness, the wafer or substrate is left with a non-planar topography as the various layers are patterned on top of one another. This type of non-planarity is often unacceptable in high density devices because the depth of field of the lithographic equipment that is used to print the smaller line width circuits on the wafer does not have a depth of focus sufficient to compensate for even small variations in wafer planarity.
In addition to the non-planarity caused by the fabricated device patterns, in-process wafer polishing, or planarization, must account for variations in overall wafer flatness as well. During the fabrication process, for example, the wafers may become bowed or warped.
In process polishing equipment, therefore, requires the specialized ability to achieve global, uniformly planar wafer surfaces in spite of these topographical wafer defects and variations. Chemical-mechanical polishing has gained wide acceptance as an effective means of achieving the global wafer surface planarity required by advanced devices employing multilayer metalization.
A typical chemical-mechanical polishing arrangement includes a wafer carrier having a generally circular pressure plate or carrier platen that supports a single substrate or wafer. A carrier film may be interposed between the carrier platen and the wafer. The wafer carrier is equipped with means to provide a downward force, urging the wafer against a polishing media (typically a circular pad), onto which is fed a polishing fluid. The polishing media is supported by a polishing platen. The polishing fluid may comprise a colloidal suspension of an abrasive and may also comprise a chemically reactive solution. A containment ring generally surrounds the wafer to prevent it from slipping off the carrier platen during polishing.
Typically, movement of the wafer relative to the pad, in the presence of the chemically reactive and/or abrasive polishing fluid and under pressure imparted by the wafer carrier, imparts a combination of chemical and mechanical forces to the wafer, the net effect of which is global planarization of the wafer surface. Generally, the polishing platen is rotatable as is the carrier platen. In a typical polishing apparatus, movement of the wafer relative to the pad is accomplished by rotating the polishing platen, the carrier platen, or both.
Rotating platen machines typically install a circular polishing pad and use it until the pad fails to obtain acceptable results because the pad becomes worn or becomes glazed with impacted polishing fluid and polishing particulate. At that time it is required to interrupt the polishing process and change the polishing pad. Other machines may use a rectangular pad or a continuous supply of polishing pad material that may be incrementally advanced over the polishing platen, to ensure that the polishing pad is never too worn to be effective.
Regardless of the configuration of the polishing pad, a common problem that occurs when the pad is not fixed to the polishing platen with an adhesive or other fixing means, is that the polishing pad migrates from its position when polishing forces are applied to it by the wafer carrier through the wafer. This migration results because the frictional forces between the wafer and the polishing pad, together with any chemical polishing media that might be employed, are greater than the frictional forces that exist between the polishing pad and the polishing platen. Such migration reduces the productivity and that accuracy of the polishing process requiring at least a reduction in the polishing pressure used in the process, thereby increasing the polishing time. Worse, the polishing pad may buckle during migration, resulting in nonplanar polishing results or total failure of the process (e.g. breakage of the substrate). These problems are not solely limited to chemical mechanical polishing but may also occur in purely mechanical polishing processes.
A problem that occurs particularly in chemical mechanical polishing machines is depletion of the chemical fluid or slurry between the substrate to be polished, and the polishing pad after a certain amount of polishing motion has occurred. Because of the relatively smooth and planar surfaces that comprise the polishing pad/platen and the substrate surface being polished, the polishing action tends to xe2x80x9csweep outxe2x80x9d the chemical fluid/slurry and a vacuum or suction builds up between the substrate surface being polished and the polishing pad. Thus, this problem gets progressively worse with polishing time. Ironically, the problem also magnifies as the surface of the substrate becomes more planar and smooth, although the problem reduces the polishing efficiency and performance of the process.
It would be desirable to have an apparatus with the capability to prevent migration of the polishing pad, while at the same time allowing easy and quick replacement either continuously or intermittently. It would also be desirable to prevent the elimination of the chemical polishing agent, e.g., the phenomena known as xe2x80x9cslurry starvationxe2x80x9d between the substrate surface to be polished and the polishing pad.
The present invention addresses the above problems, among other advantages to be further developed in the following detailed description. An apparatus for stabilizing a polishing surface includes a substantially planar inflexible support base, a flexible polishing pad overlying the support base, and a vacuum port in the support base and underlying the polishing pad. When vacuum is applied through the vacuum port, the polishing pad is pulled against the support base to maintain the polishing pad in a predetermined polishing position. A seal is preferably provided to form a perimeter around the vacuum port on the support base. An airtight seal is formed between the support base and the polishing pad along the seal upon application of the vacuum.
Another embodiment employs a plurality of vacuum ports to more evenly distribute the application of vacuum. For example, the plurality of vacuum ports may lie around a perimeter of the support base. Alternatively, the plurality of vacuum ports may be substantially equally distributed over the support base. The plurality of vacuum ports may include a group of relatively larger vacuum ports around a perimeter of the support base and a group of relatively smaller vacuum ports substantially equally distributed over an area of the support base within the perimeter.
The present invention further includes the placement of an additional layer between the polishing pad or media and the support base. The additional layer is provided so that, when the polishing pad is drawn against the support base by application of vacuum, nonplanarities are formed in the polishing pad. These nonplanarities help retain polishing fluid in the polishing area of the polishing pad during polishing of a substrate.
The additional layer is preferably a resilient pad. The additional layer is perforated so that portions of the polishing pad are partially drawn into the perforations upon the application of vacuum. Preferably, the perforations have a diameter or size in the range of about 0.06 to about 0.25 inches.
As a means for fixing a polishing pad during polishing, a polishing apparatus according to the present invention includes a substantially planar inflexible support base, a flexible polishing pad overlying the support base, and means for drawing the polishing pad against the support base to maintain the polishing pad in a predetermined polishing position. Preferably, the drawing means comprise a vacuum source connected to at least one vacuum port in the support base, but other arrangements such as a source to draw the polishing pad to the head may be alternatively used as discussed below. Further, a substrate carrier is movably mounted with respect to the polishing pad and the support base.
A pad may be interposed between the flexible polishing pad and the support base. Preferably, the pad is perforated or porous. More preferably, the pad is a perforated resilient pad.
Also disclosed is a method of stabilizing a polishing media during polishing of a substrate against the polishing media. The method includes providing a polishing media over a substantially inflexible support surface, and applying a vacuum between the support surface and the polishing media to draw the polishing media toward the support surface. The vacuum is applied with a force of about 0.2 to 3.0 psi, preferably about 1.2 psi.
The method preferably further includes polishing a substrate against the polishing media while maintaining the vacuum to deter movement of the polishing media with respect to the support surface. Still further, the method may include removing the substrate from contact with the polishing media, discontinuing application of the vacuum and removal of the polishing media for cleaning, conditioning or replacement.
The method may include replacing the same polishing media after cleaning and/or conditioning it. After replacing the same polishing media, the vacuum is reapplied between the support surface and the polishing media to draw the polishing media toward the support surface. Alternatively, another polishing media may be positioned on the support surface to replace the original polishing media. Once the new polishing media is positioned, the vacuum is then reapplied between it and the support surface to draw it toward the support surface.
Optionally, a porous layer may be provided between the polishing media and the substantially inflexible support surface prior to applying the vacuum. Then, upon application of the vacuum, portions of the polishing media are partially drawn into porosities of the porous layer to form dimples on a polishing surface of the polishing media.
An apparatus for stabilizing a polishing surface is disclosed to include a substantially inflexible support base; a flexible polishing pad overlying at least a portion of the support base; and a carrier for applying a substrate against the polishing pad to polish the substrate. Means for maintaining the flexible polishing pad substantially wrinkle free with respect to the substrate may include a vacuum port in the support base and underlying the polishing pad, or a mechanism for drawing the flexible polishing pad against the carrier and the substrate, for example.
When at least one vacuum port is provided in the support base, application of vacuum therethrough pulls the polishing pad against the support base to maintain the polishing pad in a predetermined polishing position. When the means for maintaining includes a mechanism for drawing the flexible polishing pad against the carrier and the substrate, at least one vacuum port may be formed in the carrier. A vacuum source is provided for applying a vacuum between the carrier and the polishing pad via the at least one vacuum port, thereby maintaining the polishing pad substantially wrinkle-free, at least in the portion underlying the carrier and substrate at any given time.